Heavy hydrocarbon oils sometimes contain solid or semisolid substances called sludge. Sludge formation is frequently observed when residual oils produced under severe conditions from a desulfurization apparatus, a hydrocracking apparatus, a thermal cracking apparatus, a catalytic cracking apparatus, etc., or heavy hydrocarbon oil-based mixed oils of these residual oils are preserved under improper conditions. In particular, in the recent use of cracking residues as a base for fuel oils, generation of sludge has caused serious problems in terms of the practical use of the fuel oils. These problems include obstruction of a strainer and formation of a deposit in the tip of an oil burner. The stability of fuel oils has thus been a subject of global interest.
In order to prevent these problems, a technique of rapid and precise determination of the sludge content in heavy hydrocarbon oils must be established so that the stability of heavy hydrocarbon oils can be judged for quality control.
The cause of sludge formation is considered as follows. Heavy hydrocarbon oils generally contain high-molecular weight aromatic condensed polycyclic components called "asphaltene". Asphaltene generally adsorbs aromatic components of maltene having relatively high molecular weights so as to form stable micelles. Asphaltene is an insoluble matter formed upon addition of n-heptane to heavy hydrocarbon oils. Maltene is the component soluble in n-heptane. It is considered that the stable micelles of asphaltene in the form of a colloidal dispersion lose their balance upon undergoing admixture with other fuels, thermal influence, oxidation, and the like, i.e., the micelles agglomerate and flocculate, whereby the stability of the heavy hydrocarbon oils is deteriorated and a sludge is formed. The sludge is therefore a solid or semisolid substance formed by agglomeration of aromatic components having a high condensation degree that are present in the heavy hydrocarbon oils, without forming stable micelles. Sludge is different from asphaltene which is precipitated on addition of n-heptane to heavy hydrocarbon oils. Thus, the sludge possesses different properties form those of asphaltene.
Conventional test methods for evaluating the stability of heavy hydrocarbon oils include a xylene equivalent test method, a spot test, etc., but they are not applicable to determination of sludge content. Moreover, these methods involve complicated operations, and the results obtained lack precision.
Since there is no officially standardized method of sludge determination, a Shell hot filtration test method, SMS 2696-83, which is an internally specified standard of Shell Oil Co. (hereinafter referred to as the "Shell hot filtration method") has been commonly employed for sludge determination. The Shell hot filtration method comprises pouring a sample oil on a filter fitted to a strainer of a double jacket heating system heated to 100.degree. C. so as to effect filtration under reduced pressure until no oily content is noted on the filter paper, washing the solid to semisolid substance collected on the filter with n-heptane, drying the collected substance for a given period of time, followed by weighing to obtain a dry sludge content, and determining a sludge concentration in the sample oil. However, this method, which is in the nature of a filter cake weighing method, involves complicated and troublesome operations, requires about 2 hours for obtaining the test results, and exhibits poor reproducibility. Besides, this method is unapplicable to those oils having high viscosities of 700 cSt or higher at 50.degree. C. because of the extended time required for filtration.
In the light of the latest petroleum situation, developments and studies on the lightening of heavy hydrocarbon oils have been intensively continued. Under these circumstances, it has been desired to develop a technique for rapidly determining a sludge content of heavy hydrocarbon oils, an important parameter for evaluating the stability of heavy hydrocarbon oils and heavy hydrocarbon oil-based mixed oils, and a device for carrying out such a determination.